This application claims priority of European Patent Application No. 98200010.1, which was filed on Jan. 6, 1998.
1. Field of the Invention
This invention relates to communication systems and, more particularly, OFDM (Orthogonal Frequency Division Multiplexing) modulation schemes.
2. Description of Related Art
OFDM is a block-oriented modulation scheme that maps N data symbols into N orthogonal sub-carriers separated by a frequency interval of 1/T, where T is the symbol duration, i.e. the time period over which the sub-carriers are orthogonal. As such, multi-carrier transmission systems use OFDM modulation to send data bits in parallel over multiple sub-carriers (also called tones or bins). An important advantage of multi-carrier transmission is that inter-symbol interference due to signal dispersion (or delay spread) in the transmission channel can be reduced or even eliminated by inserting a guard time interval TG between the transmission of subsequent symbols, thus avoiding an equalizer as required in single carrier systems. This gives OFDM an important advantage over single carrier modulation schemes. The guard time allows delayed copies of each symbol, arriving at the receiver after the intended signal, to die out before the succeeding symbol is received. OFDM""s attractiveness stems from its ability to overcome the adverse effects of multi-channel transmission without the need for equalization.
The transformations between blocks of symbols and base-band carrier signal are normally carried out using fast Fourier transform (FFT) techniques. A discussion of OFDM is given by Alard and Lasalle, EBU Technical Review, no. 224, August 1987, pages 168-190.
A need exists for a flexible OFDM system which provides the advantages of OFDM to a variety of communication environments.
In a previous patent application (U.S. Ser. No. 08/834,684, herein referred to as VN) I disclosed several techniques to scale data rates using OFDM. Scaling methods involve changing the clock rate, FFT size, coding rate, constellation size and guard time.
The present invention is intended to provide fallback rates with a minimum change in hardware.
The invention is an orthogonal frequency division multiplex communications apparatus employing a set of sub-carriers which are orthogonal over a time T, information-carrying symbols being expressed by superpositions of the sub-carriers, comprising
the apparatus is configured to selectively operate in one of a plurality of signaling modes in each of which the duration of each the symbol is KT where K is a positive integer, different ones of the the modes having different values of K but the same set of sub-carriers.
In a preferred embodiment of the present invention, a first signaling mode (the xe2x80x98normalxe2x80x99 mode) uses a symbol length T, a guard time TG and a set of N sub-carriers and a second mode (the xe2x80x98fallbackxe2x80x99 mode) uses a symbol length KT, a guard time KTG and the same set of N sub-carriers, where K is an integer greater than unity.
The technique can increase the range and delay spread tolerance without substantially changing the bandwidth and without changing the FFT size, at the cost of a decreased bit rate. Further, the fallback rates can also be used to provide a multiple access capability, so using fallback rates does not necessarily result in a bad spectral efficiency.